Magnets and magnetism - Generators | en - 221 - 21423
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Module 1: Magnets and magnetism

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Physics - Generators

Generators

Consider a loop that is made to spin in a magnetic field as shown. The
loop is initially lying in the magnetic field such that the flux value is
zero. When the side AB moves downwards the loop moves to a position of
maximum flux. The rate of rotation determines the rate of change of flux,
and hence the magnitude of the induced EMF. If the value of the flux was
plotted over a period of one cycle it would vary as shown. The flux value
varies in the form of a sine wave. Since the EMF is given by the rate of
change of flux, the EMF corresponds to the (negative) gradient of the
flux-time graph.

The direction of the induced voltage, or current, can be determined via an
adaptation of the right-hand slap rule. Consider one side of the loop at a
time, the side AB for example. Align the fingers with the magnetic field
lines as usual but face the palm in the opposite direction to that in which
the length of wire is moving. The thumb will then indicate the direction in
which conventional current flows.

Real generators such as the ones used in Victoria's thermal and
hydro-electric power stations work on the principles described earlier.
They generate a voltage by producing a changing magnetic flux. These
generators produce AC voltages in values of around 20 000 volts and output
hundreds of megawatts of power. Instead of having a set of loops spinning
in a stationary magnetic field, large generators often employ spinning
electromagnets, and the loops of wire remain stationary.

Victoria's brown coal power stations use burning coal to produce steam
which turns large turbines. These turbines then make the electromagnets
spin and an EMF is induced in the surrounding sets of wire loops. The
turbines must be made to spin at 3000 revolutions per minute to produce a
consistent 50 Hz AC electricity supply.

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